This invention relates to rubber articles suitable for forming weldable joints or seals to other articles. Other aspects relate to the methods of making such articles, methods in which they are jointed to other articles and the resulting composite products. The invention is particularly but not exclusively concerned with rubber components used in waterproof garments such as drysuits, which are used in diving and other watersports
By way of background, drysuits are conventionally made up from panels of strong woven fabric which has been rendered impermeable by treatment with nylon, neoprene, polyurethane or other suitable elastomer. The shaped impermeable fabric panels are joined edge-to-edge by stitching to make up the drysuit. The stitched seams are then sealed by laying hot-melt tape along them and applying heat to melt the tape. Hot-melt tapes are well-known commodities. The impermeable fabric of the suit stretches only slightly and is therefore incapable of forming seals around the wearer""s ankles, hands and neck or waist. So, soft rubber ""seals"" are provided on the suit at these positions. The conventional seal takes the form of a tubular cuff or collar of natural rubber, with a narrower opening to fit sealingly around the body (neck, arm or leg) and a wider opening at the opposite end around which it is joined sealingly to the edge of the suit""s body, sleeve or leg opening. This joint must be sealed and strong. Conventionally it is made using a solvent-based adhesive. The rubber cuff as manufactured has poor adhesion, so its margin is roughened by abrasion before applying the adhesive. There are substantial practical difficulties in aligning the adhesive-bearing cuff and suit margins before they are pressed together, because they cannot be allowed to touch until they are properly positioned.
It would be desirable to have a more convenient way of joining such rubber articles.
Broadly, we propose to provide a rubber article with an adhered coating comprising thermoplastic polymer, such as polyurethane, whereby it is weldable. The rubber article having the coating can then be conveniently non-adhesive when cold. This facilitates aligning it relative to another article to which it is to be joined without the inconvenience of premature sticking. The join may be to another thermoplastic surface, e.g. another rubber article similarly coated, a thermoplastics substrate, or to a hot-melt tape used for joining and/or sealing.
Preferably, the thermoplastic-containing coat is on a flexible rubber layer which either constitutes or is part of the rubber article. The invention has particular application to dipped rubber articles, characterized by flexible layer or membrane structure. Natural rubber is preferred. Usually the dipped rubber layer is up to (i.e. not more than) 1.5 mm thick.
In one particular context of a seal (cuff or collar) for joining around an opening of a garment, a thermoplastic coating is provided around at least a margin of the rubber seal and is overlapped with the margin of the garment fabric. For extra strength this join may be stitched. Thermoplastic polymeric tape can then be positioned along the joint to overlap the coated margin of the rubber article and the bordering region of the suit, and welded to both by the use of heat, e.g. by pre-heating the tape, or by RF treatment: these are well-established techniques. Where the joint has been stitched, the tape should be positioned to overlie the stitching.
In one embodiment a rubber article is coated only at a marginal region, where it is to be joined or sealed to another article. In an alternative embodiment a rubber article may be covered substantially or entirely with the thermoplastic coating. In the context of seals, for example, we have found that such a coating offers advantages independently of ease in joining and sealing. A seal fully coated with an adherent polyurethane or other coating can have a much longer lifetime than an uncovered rubber seal. The coating material can easily be pigmented to block light transmission, which is a primary cause of rubber deterioration. It also becomes easy to provide coloured rubber articles since pigmenting of the applied thermoplastic coating is easy, and enables a wider range of colours to be exploited than when colouring the rubber compound itself.
The thermoplastic coating polymer can be selected according to the material to which the rubber article is to be adhered, and the desired welding conditions. Polyurethanes have been found particularly suitable. Ethylene and vinyl polymers and copolymers may be used. For example a self-crosslinking ethylene-vinylchloride copolymer has been found effective, and can be applied by dipping into a latex thereof. We have used the latex FP882 (Formulated Polymer Products Ltd) with satisfactory results. More than one polymer coat may be applied if wished, and they need not be of the same polymer material. Other suitable materials for the thermoplastic layer include acrylic polymers and acrylic copolymers e.g. with vinyl comonomers, such as styrene acrylic esters. Suitable styrene acrylic ester emulsions are available as REVACRYL from Harco (Harlow Chemical Co).
The coating polymer may be entirely thermoplastic, but it is also possible to use blends. One particular option herein is to blend a thermoplastics resin with a rubber, in particular a rubber (e.g. natural rubber) the same as or similar to that of the basis article, since such a thermoplastic/rubber blend usually has better adhesion to rubber than a thermoplastics-only layer. The proportion of thermoplastics to rubber needs to be sufficient to provide weldability, if that is the envisaged end use. While this cain be determined for given polymer combinations by routine trials, as a general guide a thermoplastics/rubber polymer blend for the coating will usually contain at least 20 wt % thermoplastics (as a percentage of total polymer solids) As mentioned, it can be difficult to adhere to rubber, particularly the natural rubber from which articles such as the above-mentioned seals are often made. A rubber surface has a natural tack, but this is usually eliminated for better handling and use of the material. Conventional measures for reducing tack include applying talc or silicone to the surface, or halogenating the surface in a bath containing chlorine or bromine.
We have found certain methods particularly suited to achieving a strong, direct bond of a thermoplastics layer to the underlying rubber.
Firstly, we have discovered that chemically activating the rubber surface can be used to enable an applied thermoplastic-containing polymer coating to adhere well to rubber. In particular we have found that a halogenation treatment, such as has previously been used only for the purpose of eliminating tack, makes the rubber surface susceptible to direct adhesion of a weldable plastics coating such as a polyurethane coating.
One embodiment of the present proposal is therefore a process in which a rubber article is subjected to a halogenation of its surface followed by coating that surface with a thermoplastic, weldable polymer. The polymer is preferably a polyurethane.
The halogenation treatment may be in an aqueous halogen (chlorine or bromine)-containing bath. Such treatments are known as such, although we believe that their use to promote adhesion of a thermoplastic coating to rubber is new. The halogenation is believed to reduce or eliminate unsaturations at the rubber surface. Coating with the thermoplastic polymer is preferably by dipping, e.g. into an emulsion of the polymer.
Two other preferred process embodiments particularly exploit dipping, using latices of the rubber and of the thermoplastic resin used to form the coating.
In a second method we use a chemical coagulant such as is already routinely used for forming natural rubber articles by dipping. Latices of the rubber and of the thermoplastics are provided, each adapted (in a manner which may be well-known in itself) to be coagulated by a certain type of coagulant, e.g. a heavy metal salt such as calcium nitrate. As conventional, the coagulant is applied to a former e.g. by dipping the former into a solution of coagulant. The coagulant-carrying former is dipped into the rubber latex where the coagulant xe2x80x9cbreaks throughxe2x80x9d (i.e. coagulates) the latex, according to well-understood practice, to form a rubber layer on the former. The former with the fresh rubber layer is then removed from the rubber latex and dipped into the resin latex where residual coagulant which has permeated the rubber layer coagulates a thin layer of thermoplastics onto the rubber surface. The respective layer thickenesses are controllable by adjusting the dipping time.
We find that this method gives a strength of bond and resistance to delamination, presumably due to some degree of intermixing or fusion at the interface of the freshly-formed rubber layer and the coagulating thermoplastics, that cannot be achieved by ordinary sequential dipping (i.e. drying the rubber layer and applying further coagulant before dipping in the thermoplastics latex).
In a third method the latices contain an additive to render them heat-sensitive. Again, this is in itself well-known. One suitable sensitizer is BASF""s xe2x80x9cLutenol 40xe2x80x9d, a polyvinyl methyl ether. The former is heated, dipped in the rubber latex to form the rubber layer to the predetermined thickness, withdrawn and dipped directly in the thermoplastics-containing latex where residual heat causes coagulation of the thermoplastics layer directly onto the freshly-formed rubber layer, with benefits of bonding strength as found for the second method.
It will be appreciated by a skilled reader that excess rubber latex is allowed to run off the rubber layer before dipping into the second latex.
The second and third methods offer rapid, easily-automated procedures giving good adhesion.
A further option is to use a latex which is a blend of a thermoplastics latex and a rubber (e.g. natural rubber) latex. Such a blend may be applied to form the coating by any of the means proposed herein. The rubber containing blend tends to adhere well to the rubber substrate. Proportions in the blend have been discussed above.
In an independent aspect of the invention, a rubber article (of any kind referred to herein) may consist of such a rubber/thermoplastics, rather than having such a blend applied as a coating on a rubber substrate. For example, dipped articles may be made by dipping solely into a blended latex. Any suitable rubber and thermoplastic materials, e.g. as discussed elsewhere in this description, may be used.
Embodiments of the invention are now described by way of example with reference to the accompanying drawings.